When female rats five together in groups, their ovarian cycles become synchronized, analogous to the menstrual synchrony observed in groups of women. Ovarian synchrony represents the behavioral control of endocrine function. Among rats, the social signals that regulate the timing of the ovarian cycle are pheromones from different phases of the ovarian cycle. These pheromones can either advance or delay ovulation. In rats, we will continue our research program that integrates studies of the neuroendocrine mechanisms underlying pheromone action with complementary studies of their behavioral function and potential adaptive advantage. In addition, our strong data base in rats has now set the stage for the first definitive test of the existence of human pheromones as mediators of menstrual synchrony among women. 1. We have conceptualized ovarian synchrony as a system of coupled oscillators and developed a powerful and biologically realistic computer model that successfully simulates ovarian synchrony in rats. This computer model has generated specific testable predictions about the neuroendocrine mechanisms of pheromone action in rats. We will use our modified jugular-cannulation technique that enables us to measure hormones repeatedly from the same female rat over several weeks to determine: (a) whether ovulatory pheromones prolong cycles by changing the timing or height of the luteinizing hormone surge (LH, the hormone that triggers ovulation); (b) when rats are sensitive to these pheromones; and (c) the neuroendocrine basis for individual differences in pheromone sensitivity and timing of the LH surge. 2. We will continue to study the function of ovarian pheromones for rats by identifying specific ways that pheromones can confer a selective advantage through their effects on synchrony of birth cycles. For example, rat litters born asynchronously have more female than male pups. We will test the effects of four correlates of birth-cycle synchrony that could produce a female-biased sex ratio, potentially identifying the first known physiological mechanism for a facultative sex ratio bias in mammals. To facilitate the analysis of behavior, we will use a computerized video analysis system. 3. We will provide the first definitive evidence that human pheromones exist that could generate menstrual synchrony among women. We will evaluate the effect of putative pheromones by measuring their effects on the time of ovulation and length of the menstrual cycle. In order to do this, we need improved precision in predicting the day of ovulation and thus have developed a new protocol for predicting ovulation in women that is based on LH levels in a single daily urine sample and, to date, has a 94% accuracy.